APIs are completely custom and can be both simple and very complex. We generally use CodeIgniter for APIs as it has a very strong history with this Tech and it's lightweight and very fast. Nevertheless, APIs can be generated with almost every platform and are truly custom. They do however need a lot of documentation and Git Hub is a good place for this.
Beta development Version 1.1a. As this is an open system there is No IP white-listing or API Key requirements, but these can easily be enabled for your personal API Framework.